1. Field of the Invention
The present invention relates to a vibration damping apparatus and methods. More specifically, the invention relates to vibration damping apparatus and methods for reducing wind induced vibration of light poles, flag poles and the like.
Light poles and similar devices are subject to wind induced vibrations which can result in costly and hazardous failures of such structures. Many parameters including pole geometry and materials, wind speed, wind gust frequency and velocity are involved in effecting pole vibration. For many years those of skill in the art were of the belief that second harmonic mode vibrations were solely responsible for structural failures of poles and vibration dampers such as that shown in applicant's U.S. Pat. No. 3,612,222 were designed so as to reduce second harmonic mode vibration. Second harmonic mode vibration of poles is generally considered to be of approximately four cycles of vibration per second. However, within recent years a number of pole failures have occurred which were not caused by second harmonic mode vibrations but were resultant from first harmonic mode vibrations which occurred at a frequency of approximately one vibration cycle per second.
The present invention in one aspect is directed to vibration dampers for poles capable of reducing first mode natural frequency vibrations.
2. Related Art
The prior art is replete with devices and methods intended to reduce pole vibration as exemplified by the following discussed prior art patents.
Applicant's earlier U.S. Pat. No. 3,612,222 discloses a pole-mounted second mode natural frequency vibration damping assembly employing a metal rod loosely fitted in a tubular member attached to the pole.
Feldberg U.S. Pat. No. 6,234,286 purports to be an improvement over applicant's earlier U.S. Pat. No. 3,612,222 with respect to which it differs only in a minor manner relating to the attachment of the device to a pole.
Kemeny U.S. Pat. No. 6,035,981 discloses a complicated broadband passive mass damper intended for use on machinery or other equipment.
Aida et al. U.S. Pat. No. 5,896,961 discloses a dynamic vibration absorber mounted on a light pole by a variety of magnetic devices.
Phillips et al. U.S. Pat. No. 5,682,069 discloses a vibration damping assembly mounted on a strut about which the device surrounds.
Hamada et al. U.S. Pat. No. 5,593,144 discloses a vibration damping device mounted on a drive shaft.
Knodo et al. U.S. Pat. No. 4,736,701 discloses a variety of devices for attachment to the upper end of a mast.
Lehmann et al. U.S. Pat. No. 4,640,216 is directed to a vibration damping assembly for a periscope.
Buckley U.S. Pat. No. 4,350,233 discloses a vibration damping device provided internally of a pole or the like effecting first harmonic mode vibration damping.
Reed U.S. Pat. No. 3,568,803 discloses a vibration damping device attached to the upper end of a mast.
Reutlinger U.S. Pat. No. 3,382,629 discloses a vibration damping assembly attached to a mast or pole.
Nishioka et al. U.S. Pat. No. 3,259,212 discloses a pole vibration damping device mounted on the outer end of a curved pole.
Chen U.S. Pat. No. 3,245,177 is directed to a vibration damping device for reducing vibrations of a chimney or the like by the use of porous, loose fill material and liquid.
Chen U.S. Pat. No. 3,174,589 is similar to the above-noted U.S. Pat. No. 3,245,177.